Shift register of lcd devices

ABSTRACT

A shift register includes a plurality of shift register units coupled in series. Each shift register unit, receiving an input voltage at an input end and an output voltage at an output end, includes a node, a pull-up driving circuit, a pull-up circuit and first through third pull-down circuits. The pull-up driving circuit can transmit the input voltage to the node, and the pull-up circuit can provide the output voltage based on a high-frequency clock signal and the input signal. The first pull-down circuit can provide a bias voltage at the node or at the output end based on a first low-frequency clock signal. The second pull-down circuit can provide a bias voltage at the node or at the output end based on a second low-frequency clock signal. The third pull-down circuit can provide a bias voltage at the node or at the output end based on a feedback voltage.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 13/492,916filed Jun. 10, 2012, which is a division of U.S. application Ser. No.12/607,042 filed Oct. 27, 2009, the entire contents of which areincluded herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to a shift register, and moreparticularly, to a shift register which drives pull-down circuits usinglow-frequency signals.

2. Description of the Prior Art

Liquid crystal display (LCD) devices, characterized in low radiation,small size and low power consumption, have gradually replacedtraditional cathode ray tube (CRT) devices and been widely used invarious electronic devices, such as notebook computers, personal digitalassistants (PDAs), flat panel televisions or mobiles phones. TraditionalLCD devices display images by driving the panel pixels using externaldriver chips. In order to reduce the number of devices and manufacturingcosts, GOA (gate driver on array) technique has been developed in whichthe gate driver is directly manufactured on the LCD panel. A GOA gatedriver usually functions in form of shift register, in which pull-upcircuits provide a plurality of gate driving signals to the displaypanel and pull-down circuits stabilize output signals.

Referring to FIG. 1, which depicts a simplified functional diagram of aprior art LCD device 100. FIG. 1 only illustrates partial structure ofthe LCD device 100, including gate lines GL(1)-GL(N), a shift register110, a clock generator 120 and a power supply 130. For operating theshift register 110, the clock generator 120 provides a start pulsesignal VST and two high-frequency clock signals CH1 and CH2, while thepower supply 130 provides bias voltages VDD and VSS. The shift register110 includes a plurality of serially-coupled shift register unitsSR(1)-SR(N) having output ends coupled to corresponding gate linesGL(1)-GL(N), respectively. According to the high-frequency clock signalsCH1, CH2 and the start pulse signal VST, the shift register 110 cansequentially output gate driving signals GS(1)-GS(N) to thecorresponding gate lines GL(1)-GL(N) via the shift register unitsSR(1)-SR(N), respectively.

Referring to FIG. 2, which depicts a diagram of a shift registerillustrated in U.S. Pat. No. 7,310,402 “GATE LINE DRIVERS FOR ACTIVEMATRIX DISPLAYS” (hereinafter as “prior art 1”). FIG. 2 illustrates acircuit diagram of an nth stage shift register unit SR(n) among theplurality of shift register units SR(1)-SR(N) in prior art 1(n is aninteger between 1 and N). The shift register unit SR(n) includes aninput end IN(n), an output end OUT(n), a first pull-down circuit 16, asecond pull-down circuit 26, a maintain circuit 36, a pull-up drivingcircuit 46, and a pull-up circuit 56. The input end IN(n) of the shiftregister unit SR(n) is coupled to the output end OUT(n−1) of theprior-stage shift register unit SR(n−1), while the output end OUT(n) ofthe shift register unit SR(n) is coupled to the input end IN(n+1) of thenext-stage shift register unit SR(n+1). In prior art 1, the pull-upoperation is performed by transistor switches T1 and T2, wherein thetransistor switch T1 controls the signal transmission path between theinput end IN(n) and the node Q(n) according to the gate driving signalGS(n−1), and the transistor switch T2 controls the signal transmissionpath between the clock signal CH1 and the output end OUT(n) according tothe voltage level of the node Q(n). Meanwhile, the pull-down operationis performed by the first pull-down circuit 16 and the second pull-downcircuit 26. In the first pull-down circuit 16, the serially-coupledtransistor switches T3 and T4 respectively receive the high-frequencyclock signals CH1 and CH2 having opposite phases at their respectivegates, thereby generating control signals to the gates of the transistorswitches T5 and T6. Therefore, the transistor switch T5 can control thesignal transmission path between the node Q(n) and the voltage VSSaccording to its gate voltage, and the transistor switch T6 can controlthe signal transmission path between the output end OUT(n) and thevoltage VSS according to its gate voltage. In the second pull-downcircuit 26, the serially-coupled transistor switches T7 and T8respectively receive the high-frequency clock signals CH2 and CH1 havingopposite phases at their respective gates, thereby generating controlsignals to the gates of the transistor switches T9 and T10. Therefore,the transistor switch T9 can control the signal transmission pathbetween the node Q(n) and the voltage VSS according to its gate voltage,and the transistor switch T10 can control the signal transmission pathbetween the output end OUT(n) and the voltage VSS according to its gatevoltage. The maintain circuit 36 maintains the gate voltages of thetransistor switches T5, T6, T9 and T10 using the transistor switchesT11-T13.

Referring to FIG. 3, which depicts a diagram of a shift registerillustrated in U.S. Pat. No. 7,342,568 “SHIFT REGISTER CIRCUIT”(hereinafter as “prior art 2”). FIG. 3 illustrates a circuit diagram ofan nth stage shift register unit SR(n) among the plurality of shiftregister units SR(1)-SR(N) in prior art 2 (n is an integer between 1 andN). The shift register unit SR(n) includes an input end IN(n), an outputend OUT(n), a first pull-down circuit 18, a second pull-down circuit 28,a third pull-down circuit 38, a pull-up driving circuit 48, and apull-up circuit 58. The input end IN(n) of the shift register unit SR(n)is coupled to the output end OUT(n−1) of the prior-stage shift registerunit SR(n−1), while the output end OUT(n) of the shift register unitSR(n) is coupled to the input end IN(n+1) of the next-stage shiftregister unit SR(n+1). In prior art 2, the pull-up operation isperformed by transistor switches T1 and T2, wherein the transistorswitch T1 controls the signal transmission path between the input endIN(n) and the node Q(n) according to the gate driving signal GS(n−1),and the transistor switch T2 controls the signal transmission pathbetween the clock signal CH1 and the output end OUT(n) according to thevoltage level of the node Q(n). Meanwhile, the main pull-down operationis performed by the first pull-down circuit 18 and the second pull-downcircuit 28. In the first pull-down circuit 18, the serially-coupledtransistor switches T3 and T4 respectively receive the high-frequencyclock signals CH1 and CH2 having opposite phases at their respectivegates, thereby generating control signals to the gates of the transistorswitches T5 and T6. Therefore, the transistor switch T5 can control thesignal transmission path between the node Q(n) and the voltage VSSaccording to its gate voltage, and the transistor switch T6 can controlthe signal transmission path between the output end OUT(n) and thevoltage VSS according to its gate voltage. In the second pull-downcircuit 28, the serially-coupled transistor switches T7 and T8respectively receive the high-frequency clock signals CH2 and CH1 havingopposite phases at their respective gates, thereby generating controlsignals to the gates of the transistor switches T9 and T10. Therefore,the transistor switch T9 can control the signal transmission pathbetween the node Q(n) and the voltage VSS according to its gate voltage,and the transistor switch T10 can control the signal transmission pathbetween the output end OUT(n) and the voltage VSS according to its gatevoltage.

Referring to FIG. 4, which depicts a timing diagram illustrating theoperation of the prior art shift register. When driving the LCD devicesin prior art 1 and prior art 2, the clock signals CH1 and CH2 switchbetween a high voltage level Vgh and a low voltage level Vg1 based on aduty cycle of 50%, and have opposite phases at the same time. The 1ststage shift register unit SR(1) generates the 1st stage gate drivingsignal GS(1) according to the start pulse signal VST, while the 2nd-Nthstage shift register units SR(2)-SR(N) generate the 2nd-Nth stage gatedriving signals GS(2)-GS(N) according to the output signals generated bytheir respective prior-stage shift register units. In other words, thegate driving signals GS(2)-GS(N) can be viewed as the start pulsesignals which respectively enable the shift register units SR(2)-SR(N).The prior art shift register performs the pull-up operation between t1and t3, and performs the pull-down operation after t3. For the nth stageshift register unit SR(n), the period between t1 and t2 is the drivingperiod of its prior-stage shift register unit SR(n−1), during which theclock signal CH1 is at low voltage level, while the clock signal CH2 andthe gate driving signal GS(n−1) are at high voltage level. Thetransistor switch T1 is thus turned on, thereby pulling up the node Q(n)to the high voltage level VDD. Meanwhile, the transistor switch T2 isalso turned on, and the gate driving signal GS(n) is pulled down to thelow voltage level Vg1 due to the feed-through effect of the transistorswitches in the pixels. At t2, the clock signal CH1 switches from lowvoltage level to high voltage level, thereby providing the gate drivingsignal GS(n) having high voltage level between t2 and t3 (when the clocksignal CH1 is at high voltage level) via the turned-on transistor switchT2. On the other hand, the pull-down circuits 16, 26 and 18, 28 operatein a complimentary manner and each handles 50% of the pull-downoperation. The gate driving signal GS(n) can thus be maintained at lowvoltage level VSS during the time excluding the driving period of thenth stage shift register unit SR(n). When the clock signal CH1 is at lowvoltage level, the clock signal CH2 is at high voltage level, the inputsignal of the shift register unit SR(n) (the gate driving signalGS(n−1)) is at low voltage level, and the output signal of the shiftregister unit SR(n) (the gate driving signal GS(n)) is at low voltagelevel, the gates of the transistor switches T5 and T6 are substantiallykept at the low voltage level VSS, while the gates of the transistorswitches T9 and T10 are substantially kept at the high voltage levelVDD. Similarly, when the clock signal CH1 is at high voltage level, theclock signal CH2 is at low voltage level, and the output signal of theshift register unit SR(n) (the gate driving signal GS(n)) is at lowvoltage level, the gates of the transistor switches T5 and T6 aresubstantially kept at the high voltage level VDD, while the gates of thetransistor switches T9 and T10 are substantially kept at the low voltagelevel VSS. Therefore, in the prior art shift register, the gates of thetransistor switches T5, T6, T9 and T10 are kept at high voltage levelfor about 50% of a period, while at low voltage level for about 50% ofthe period.

With increasing demand for panel resolution, the charge time of thepixels is shortened. The clock signals CH1 and CH2 need to have higherfrequencies, and power consumption also increases accordingly. In theprior art LCD devices, the high-frequency clock signals CH1 and CH2 areused for driving the pull-down circuits. In addition to high powerconsumption, the pull-down operation might eventually fail as thecharacteristics of the transistor switches deviate with time. Meanwhile,due to the feed-through effect, the gate driving signal GS(n) isdischarged to the low voltage level Vg1 which is lower than the ideallevel VSS before its driving period, thereby causing charge coupling tothe data voltages of the pixels and influencing the display quality ofthe LCD device.

SUMMARY OF THE INVENTION

The present invention provides a shift register comprising a pluralityof serially-coupled shift register units, an Nth stage shift registerunit among the plurality of shift register units comprising an input endfor receiving an input voltage; an output end for outputting an outputvoltage; a first node; a pull-up driving circuit for transmitting theinput voltage to the first node; a pull-up circuit for providing theoutput voltage according to a first clock signal and the input voltage;a first pull-down circuit for providing a first voltage at the firstnode or at the output end according to a second clock signal; a secondpull-down circuit for providing a second voltage at the first node or atthe output end according to a third clock signal, wherein a frequency ofthe first clock signal is higher than a frequency of the second or thethird clock signal; and a third pull-down circuit for providing a thirdvoltage at the first node or at the output end according to a feedbackvoltage.

The present invention further provides a shift register comprising aplurality of serially-coupled shift register units, an Nth stage shiftregister unit among the plurality of shift register units comprising aninput end for receiving an input voltage; an output end for outputtingan output voltage; a node; a pull-up driving circuit for transmittingthe input voltage to the node; a pull-up circuit for providing theoutput voltage according to a first clock signal and the input voltageso that the output voltage is at a first voltage level during a drivingperiod of the Nth stage shift register unit, wherein the first clocksignal switches between the first voltage level and a second voltagelevel with a predetermined frequency and the first voltage level ishigher than the second voltage level; a pull-down circuit formaintaining the output voltage at a third voltage level during a periodexcluding the driving period of the Nth stage shift register unit,wherein the third voltage level is higher than the second voltage level;and a fast pull-down circuit for maintaining a voltage level of the nodeor the output end according to a feedback voltage so that the outputvoltage is at the second voltage level during a driving period of an(N+1)th stage shift register unit among the plurality of the shiftregister units.

The present invention further provides a shift register comprising aplurality of serially-coupled shift register units, an Nth stage shiftregister unit among the plurality of shift register units comprising aninput end for receiving an input voltage; an output end for outputtingan output voltage; a node; a pull-up driving circuit for transmittingthe input voltage to the node; a pull-up circuit for providing theoutput voltage according to a first clock signal and the input voltageso that the output voltage is at a second voltage level during a drivingperiod of an (N−1)th stage shift register unit, at a first voltage levelduring a driving period of the Nth stage shift register unit, and at thesecond voltage level during a driving period of an (N+1)th stage shiftregister unit, wherein the first clock signal switches between the firstand second voltage levels with a predetermined frequency and the firstvoltage level is higher than the second voltage level; and a pull-downcircuit for maintaining the output voltage at a third voltage levelduring a period excluding the driving period of the Nth stage shiftregister unit, wherein the third voltage level is higher than the secondvoltage level.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified functional diagram of a prior art LCD device.

FIGS. 2 and 3 are diagrams illustrating prior art shift registers.

FIG. 4 is a timing diagram illustrating the operation of the prior artshift registers.

FIGS. 5 and 6 are simplified functional diagrams of an LCD deviceaccording to the present invention.

FIG. 7 is a diagram of an nth stage shift register unit according to afirst embodiment of the present invention.

FIG. 8 is a diagram of an nth stage shift register unit according to asecond embodiment of the present invention.

FIG. 9 is a diagram of an nth stage shift register unit according to athird embodiment of the present invention.

FIG. 10 is a diagram of an nth stage shift register unit according to afourth embodiment of the present invention.

FIG. 11 is a timing diagram illustrating the operation of the shiftregister according to the first through fourth embodiments of thepresent invention.

FIG. 12 is a diagram of an nth stage shift register unit according to afifth embodiment of the present invention.

FIG. 13 is a diagram of an nth stage shift register unit according to asixth embodiment of the present invention.

FIG. 14 is a diagram of an nth stage shift register unit according to aseventh embodiment of the present invention.

FIG. 15 is a diagram of an nth stage shift register unit according to aneighth embodiment of the present invention.

FIGS. 16-18 are timing diagrams illustrating the operation of the shiftregister according to the fifth through eighth embodiments of thepresent invention.

DETAILED DESCRIPTION

Referring to FIGS. 5 and 6, which depict simplified functional diagramsof an LCD device 300 according to the present invention, including gatelines GL(1)-GL(N), a shift register 210, a clock generator 220 and apower supply 230. For operating the shift register 210, the clockgenerator 220 provides start pulse signals VST/VST1/VST2, a plurality ofhigh-frequency clock signals CH1-CHM, and two low-frequency clocksignals CL1 and CL2, while the power supply 230 provides a bias voltageVSS. The shift register 210 includes a plurality of serially-coupledshift register units SR(1)-SR(N). According to correspondinghigh-frequency clock signals CH1-CHM, corresponding input signalsST(1)-ST(N−1) and corresponding feedback signals FB(1)-FB(N), the shiftregister units SR(1)-SR(N) can sequentially output gate driving signalsGS(1)-GS(N) to the corresponding gate lines GL(1)-GL(N) at respectiveoutput ends OUT(1)-OUT(N). For the 1st stage shift register unit SR(1),the input signal ST(1) is the start pulse signal VST, VST1 or VST2provided by the clock generator 220, and the received feedback signalFB(2) is the gate driving signal GS(2) generated by the 2nd stage shiftregister unit SR(2). For an nth stage shift register unit SR(n) amongthe shift register units SR(2)-SR(N), its input end IN(n) is coupled toa prior-stage shift register unit SR(n−m) and its output end OUT(n) iscoupled to the input end of a next-stage shift register unit SR(n+m).Therefore, the input signal ST(n−m) is provided by the shift registerSR(n−m), and the feedback signal FB(n+m) is the gate driving signalGS(n+m) provided by the shift register SR(n+m), wherein each of (n+m)and (n+m) is a positive integer smaller than N, and the value of M isequal to 2^(m).

FIG. 5 depicts a simplified functional diagram of the LCD device 300according to the present invention when m=1, while FIG. 6 depicts asimplified functional diagram of the LCD device 300 according to thepresent invention when m=2. In the embodiment illustrated in FIG. 5(m=1), the 1st stage shift register unit SR(1) generates the 1st stagegate driving signal GS(1) according to the start pulse signal VST1,while an nth stage shift register unit SR(n) among the shift registerunits SR(2)-SR(N) generates the nth stage gate driving signal GS(n)according to the input signal ST(n−1) generated by the shift registerunit SR(n−1) and the feedback signal FB(n+1) generated by the shiftregister unit SR(n+1). In the embodiment illustrated in FIG. 6 (m=2),the 1st stage shift register unit SR(1) generates the 1st stage gatedriving signal GS(1) according to the start pulse signal VST or VST1,the 2nd stage shift register unit SR(2) generates the 2nd stage gatedriving signal GS(2) according to the start pulse signal VST or VST2,and an nth stage shift register unit SR(n) among the shift registerunits SR(3)-SR(N) generates the nth stage gate driving signal GS(n)according to the output signal generated by the shift register unitSR(n−2) and the feedback signal generated by the shift register unitSR(n+2).

FIGS. 5 and 6 only illustrate detailed functional diagrams of the nthstage shift register unit SR(n) and other shift register units haveidentical structure. The shift register unit SR(n) includes a firstpull-down circuit, a second pull-down circuit, a third pull-downcircuit, a pull-up driving circuit and a pull-up circuit. The input endof the shift register unit SR(n) is coupled to a prior-stage shiftregister unit SR(n−m), while the output end OUT(n) of the shift registerunit SR(n) is coupled to a next-stage shift register unit SR(n+m). Thefirst pull-down circuit operates according to the low-frequency clocksignal CL1, the second pull-down circuit operates according to thelow-frequency clock signal CL2, the third pull-down circuit operatesaccording to the gate driving signal GS(n+m) generated by the shiftregister unit SR(n+m), the pull-up driving circuit operates according tothe signal transmitted from the shift register unit SR(n−m), and thepull-up circuit operates according to a corresponding high-frequencyclock signal among the high-frequency clock signals CH1-CHM. Forexample, the pull-up circuits of the nth-(n+3)th shift register unitsSR(n)-SR(n+3) operate according to the high-frequency clock signalsCH1-CH4, respectively.

Referring to FIG. 7, which depicts a diagram of the nth stage shiftregister unit SR(n) according to a first embodiment of the presentinvention. In the first embodiment of the present invention, the shiftregister unit SR(n) includes an input end IN(n), an output end OUT(n), afirst pull-down circuit 11, a second pull-down circuit 21, a thirdpull-down circuit 31, a pull-up driving circuit 41 and a pull-up circuit51. The pull-up driving circuit 41 includes a transistor switch T1having a gate and a drain coupled to the input end IN(n) for receivingthe gate driving signal GS(n−m) from the shift register SR(n−m) and asource coupled to the node Q(n), thereby capable of controlling thesignal transmission path between the input end IN(n) and the node Q(n)according the gate driving signal GS(n−m). The pull-up circuit 51includes a transistor switch T2 having a gate coupled to the node Q(n),a drain coupled to the clock generator 220 for receiving ahigh-frequency clock signal among the high-frequency clock signalsCH1-CHM (such as CH1) and a source coupled to the output end OUT(n),thereby capable of controlling the signal transmission path between thehigh-frequency clock signal CH1 and the output end OUT(n) according thevoltage level of the node Q(n).

The first pull-down circuit 11 includes transistor switches T3-T6. Thetransistor switch T3 includes a gate coupled to the node K(n), a draincoupled to the node Q(n) and a source coupled to the output end OUT(n).The transistor switch T4 includes a gate coupled to the node K(n), adrain coupled to the output end OUT(n) and a source coupled to a voltagesource VSS which provides a negative bias voltage. The transistor switchT5 includes a gate and a drain coupled to the clock generator 220 forreceiving the low-frequency signal CL1 and a source coupled to the nodeK(n). The transistor switch T6 includes a gate coupled to the node Q(n),a drain coupled to the node K(n) and a source coupled to the voltagesource VSS. The transistor switch T5 in the first pull-down circuit 11controls the voltage level of the node K(n) according to thelow-frequency signal CL1. When the node K(n) is at high voltage level,the node Q(n) is electrically connected to the output end OUT(n) via theturned-on transistor switch T3, and the output end OUT(n) is thenelectrically connected to the voltage source VSS via the turned-ontransistor switch T4.

The second pull-down circuit 21 includes transistor switches T7-T10. Thetransistor switch T7 includes a gate coupled to the node P(n), a draincoupled to the node Q(n) and a source coupled to the output end OUT(n).The transistor switch T8 includes a gate coupled to the node P(n), adrain coupled to the output end OUT(n) and a source coupled to thevoltage source VSS. The transistor switch T9 includes a gate and a draincoupled to the clock generator 220 for receiving the low-frequencysignal CL2 and a source coupled to the node P(n). The transistor switchT10 includes a gate coupled to the node Q(n), a drain coupled to thenode P(n) and a source coupled to the voltage source VSS. The transistorswitch T9 in the second pull-down circuit 21 controls the voltage levelof the node P(n) according to the low-frequency signal CL2. When thenode P(n) is at high voltage level, the node Q(n) is electricallyconnected to the output end OUT(n) via the turned-on transistor switchT7, and the output end OUT(n) is then electrically connected to thevoltage source VSS via the turned-on transistor switch T8.

The third pull-down circuit 31 includes transistor switches T11 and T12.The transistor switch T11 includes a gate coupled to the shift registerunit SR(n+m) for receiving the feedback signal FB(n), a drain coupled tothe node Q(n), and a source coupled to the voltage source VSS. Thetransistor switch T12 includes a gate coupled to the shift register unitSR(n+m) for receiving the feedback signal FB(n), a drain coupled to theoutput end OUT(n), and a source coupled to the voltage source VSS. Thefeedback signal FB(n) received by the third pull-down circuit 31 is thegate driving signal GS(n+m) generated by the shift register unitSR(n+m). When the gate driving signal GS(n+m) is at high voltage level,both the output end OUT(n) of the shift register unit SR(n) and the nodeQ(n) is pulled down to low voltage level. When m=1, the feedback signalFB(n) is the gate driving signal GS(n+1) generated by the shift registerunit SR(n+1); when m=2, the feedback signal FB(n) is the gate drivingsignal GS(n+2) generated by the shift register unit SR(n+2), etc.

The transistor switch T10 includes a gate coupled to the node Q(n), adrain coupled to the node P(n) and a source coupled to the voltagesource VSS. The transistor switch T9 in the second pull-down circuit 21controls the voltage level of the node P(n) according to thelow-frequency signal CL2. When the node P(n) is at high voltage level,the node Q(n) is electrically connected to the output end OUT(n) via theturned-on transistor switch T7, and the output end OUT(n) is thuselectrically connected to the voltage source VSS via the turned-ontransistor switch T8.

Referring to FIG. 8, which depicts a diagram of the nth stage shiftregister unit SR(n) according to a second embodiment of the presentinvention. In the second embodiment of the present invention, the shiftregister unit SR(n) includes an input end IN(n), an output end OUT(n), afirst pull-down circuit 11, a second pull-down circuit 21, a thirdpull-down circuit 31, a pull-up driving circuit 42 and a pull-up circuit51. Having similar structure as the first embodiment, the secondembodiment of the present invention differs in that the pull-up drivingcircuit 42 includes transistor switches T1 and T13. The transistorswitch T1 includes a gate coupled to the drain of the transistor switchT13, a drain coupled to the input end IN(n) for receiving the gatedriving signal GS(n−m), and a source coupled to the node Q(n). Thetransistor switch T13 includes a gate coupled to the clock generator 220for receiving the high-frequency clock signal CHn used by the shiftregister unit SR(n−m), and a source coupled to the node Q(n−m) of theshift register unit SR(n−m). The transistor switch T13 can maintain thegate voltage of the transistor switch T1, thereby reducing the leakageof the transistor switch T1. When m=1, the gate of the transistor switchT13 is coupled to the clock generator 220 for receiving thehigh-frequency clock signal used by the shift register unit SR(n−2)(such as CH4), and the source of the transistor switch T13 is coupled tothe node Q(n−1) of the shift register unit SR(n−1); when m=2, the gateof the transistor switch T13 is coupled to the clock generator 220 forreceiving the high-frequency clock signal used by the shift registerunit SR(n−2) (such as CH3), and the source of the transistor switch T13is coupled to the node Q(n−2) of the shift register unit SR(n−2), etc.

Referring to FIG. 9, which depicts a diagram of the nth stage shiftregister unit SR(n) according to a third embodiment of the presentinvention. In the third embodiment of the present invention, the shiftregister unit SR(n) includes an input end IN(n), an output end OUT(n), afirst pull-down circuit 13, a second pull-down circuit 23, a thirdpull-down circuit 31, a pull-up driving circuit 41 and a pull-up circuit53. The first and third embodiments of the present invention havesimilar structure, but differ in the structures of the first pull-downcircuit 13, the second pull-down circuit 23, and the pull-up circuit 53.In the third embodiment of the present invention, the pull-up circuit 53includes transistor switches T2 and T14. The transistor switch T2includes a gate coupled to the node Q(n), a drain coupled to the clockgenerator 220 for receiving one of the high-frequency clock signalsCH1-CHM (such as CH1), and a source coupled to the output end OUT(n),thereby capable of controlling the signal transmission path between theclock signal CH1 and the output end OUT(n) according to the voltagelevel of the node Q(n). The transistor switch T14, functioning as acarrier buffer, includes a gate coupled to the node Q(n), a draincoupled to the clock generator 220 for receiving one of thehigh-frequency clock signals CH1-CHM (such as CH1), and a source coupledto the node H(n), thereby capable of controlling the signal transmissionpath between the clock signal CH1 and the node H(n) according to thevoltage level of the node Q(n). In the first embodiment of the presentinvention, the same gate driving signal GS(n) is transmitted to the gateline GL(n) and the shift register unit SR(n+m). However in the thirdembodiment of the present invention, the transistor switch T14 furthergenerates the input signal ST(n+m) corresponding to the gate drivingsignal GS(n). Then, the gate driving signal GS(n) and the input signalST(n+m) are transmitted to the gate line GL(n) and the shift registerunit SR(n+m), respectively. In other words, the pull-up driving circuit41 according to the third embodiment of the present invention operatesaccording to the signal ST(n−m) generated at the node H(n−m) of theshift register unit SR(n−m). Meanwhile, the first pull-down circuit 13further includes a transistor switch T15 and the second pull-downcircuit 23 further includes a transistor switch T16, thereby capable ofmaintaining the voltage level of the node H(n) according the voltagelevels of the nodes K(n) and P(n).

Referring to FIG. 10, which depicts a diagram of the nth stage shiftregister unit SR(n)according to a fourth embodiment of the presentinvention. In the fourth embodiment of the present invention, the shiftregister unit SR(n) includes an input end IN(n), an output end OUT(n), afirst pull-down circuit 13, a second pull-down circuit 23, a thirdpull-down circuit 31, a pull-up driving circuit 42 and a pull-up circuit53. The third and fourth embodiments of the present invention havesimilar structure, but the pull-up driving circuit 42 in the fourthembodiment includes transistor switches T1 and T13. The transistorswitch T1 includes a gate coupled to the drain of the transistor switchT13, a drain coupled to the input end IN(n) for receiving the signalST(n−m), and a source coupled to the node Q(n), thereby capable ofoperating according to the signal ST(n−m) generated at the node H(n−m)of the shift register unit SR(n−m). The transistor switch T13 includes agate coupled to the clock generator 220 for receiving the high-frequencyclock signal CHn used by the shift register unit SR(n−m), and a sourcecoupled to the node Q(n−m) of the shift register unit SR(n−m). Thetransistor switch T13 can maintain the gate voltage of the transistorswitch T1, thereby reducing the leakage of the transistor switch T1.When m=1, the gate of the transistor switch T13 is coupled to the clockgenerator 220 for receiving the high-frequency clock signal used by theshift register unit SR(n−1) (such as CH4), and the source of thetransistor switch T13 is coupled to the node Q(n−1) of the shiftregister unit SR(n−1); when m=2, the gate of the transistor switch T13is coupled to the clock generator 220 for receiving the high-frequencyclock signal used by the shift register unit SR(n−2) (such as CH3), andthe source of the transistor switch T13 is coupled to the node Q(n−2) ofthe shift register unit SR(n−2), etc.

Referring to FIG. 11, which depicts a timing diagram illustrating theoperation of the shift register 300 according to the first throughfourth embodiments of the present invention. In the embodimentillustrated in FIG. 5, the low-frequency clock signals CL1, CL2, thehigh-frequency clock signals CH1, CH2, and the start pulse signal VSTare used for driving the shift register 210. In the embodimentillustrated in FIG. 6, the low-frequency clock signals CL1, CL2, thehigh-frequency clock signals CH1-CH4, and the start pulse signal VST,VST1 or VST2 are used for driving the shift register 210. The shiftregister units SR(1) and SR(2) can both be enabled by the start pulsesignal VST, or by the start pulse signals VST1 and VST2, respectively.The pulse width of the high-frequency clock signals CH1-CH4 is equal tothat of the start pulse signals VST1 and VST2, but the high-frequencyclock signals CH1-CH4 differ in phase. The pulse width of the startpulse signal VST is twice as large as that of the start pulse signalVST1 or VST2. Each clock signal switches between a high voltage levelVgh and a low voltage level Vg1 based on a predetermined frequency,wherein the frequency of the low-frequency clock signals CL1 and CL2 ismuch lower than that of the high-frequency clock signals CH1-CH4 (forexample, the pulse width of the low-frequency clock signals CL1 and CL2can be around 100 times as large as that of the start pulse signal VST1or VST2, or the low-frequency clock signals CL1 and CL2 can switchphases every 100 frames when the high-frequency clock signals CH1-CH4switch phases after each frame), and the low-frequency clock signals CL1and CL2 have opposite phases at the same time. OUT(n), Q(n), K(n) andP(n) respectively represent the waveforms of the signals at the outputend OUT(n) of the nth stage shift register unit SR(n), the node Q(n),the node K(n) and the node P(n), and will be explained in detail in thefollowing paragraphs.

The high-frequency clock signal CH1, two low-frequency clock signal CL1,CL2, and the start pulse signal VST are used for driving the shiftregister unit SR(n) in the first through fourth embodiments of thepresent invention. The low-frequency clock signals CL1 and CL2 haveopposite phases: when the low-frequency clock signal CL1 is at highvoltage level, the main pull-down operation is performed by the firstpull-down circuit 11 or 13; when the low-frequency clock signal CL2 isat high voltage level, the main pull-down operation is performed by thesecond pull-down circuit 21 or 23. For the nth stage shift register unitSR(n), the low-frequency clock signal CL1 remains at high voltage leveland the low-frequency clock signal CL2 remains at low voltage levelbefore t1, during which the first pull-down circuit 11 or 13 carries outthe pull-down operation. At t1, the low-frequency clock signal CL1switches from high voltage level to low voltage level and thelow-frequency clock signal CL2 switches from low voltage level to highvoltage level. With the node P(n) being pulled up to high voltage levelvia the turned-on transistor switch T9, the transistor switches T7 andT8 are then turned on, thereby maintaining the node Q(n) and the outputend OUT(n) at low voltage level. The main pull-down operation is thusperformed by the second pull-down circuit 21 or 23 during this period inwhich the transistor switch T5 is turned off but the node K(n) is stillkept at high voltage level. Therefore, a part of the pull-down operationis still carried out by the first pull-down circuit 11 or 13. At t2, theinput signal IN(n), which is the gate driving signal GS(n−m) in thefirst trough fourth embodiments of the present invention, switches fromlow voltage level to high voltage level, thereby pulling up the nodeQ(n) to high voltage level and then turning on the transistor switchesT2, T6 and T10. Therefore, the node K(n) is pulled down to low voltagelevel via the turned-on transistor switch T6, the node P(n) is pulleddown to low voltage level via the turned-on transistor switch T10, andthe gate driving signal GS(n) is pulled down to the low voltage levelVg1 due to the feed-through effect of the transistor switches in thepixels. At t3, the nth stage shift register SR(n) begins to perform thepull-up operation. The high-frequency signal CH1 switches from lowvoltage level to high voltage level, and is transmitted to the outputend OUT(n) via the turned-on transistor switch T2, thereby providing thegate driving signal GS(n) to the gate line GL(n) and the shift registerunit SR(n+m). At t4, the nth stage shift register SR(n) completes thepull-up operation and the high-frequency signal CH1 switches from highvoltage level to low voltage level. With the output end OUT(n) beingpulled up to low voltage level, the transistor switches T7 and T8 areagain turned on and the second pull-down circuit 21 resumes performingthe pull-down operation, thereby maintaining the node Q(n) and theoutput end OUT(n) at low voltage level. At this time, the voltage levelsof the nodes K(n) and P(n) are determined by the low-frequency clocksignals CL1 and CL2, respectively. In the first through fourthembodiments of the present invention, the lifetime and accuracy of thetransistor switches can be increased by driving the shift register witha plurality of high-frequency clock signals and two low-frequency clocksignals.

Referring to FIG. 12, which depicts a diagram of the nth stage shiftregister unit SR(n) according to a fifth embodiment of the presentinvention. In the fifth embodiment of the present invention, the shiftregister unit SR(n) includes an input end IN(n), an output end OUT(n), afirst pull-down circuit 11, a second pull-down circuit 21, a fastpull-down circuit 35, a pull-up driving circuit 41 and a pull-up circuit51. Having similar structure as the first embodiment, the fifthembodiment of the present invention further includes the fast pull-downcircuit 35. The fast pull-down circuit 35 in the fifth embodiment of thepresent invention includes transistor switches T11 and T12. Thetransistor switch T11 includes a gate coupled to the output end OUT(n+s)of the shift register unit SR(n+s) for receiving the feedback signalFB(n), a drain coupled to the node Q(n), and a source coupled to thevoltage source VSS. The transistor switch T12 includes a gate coupled tothe gate of the transistor switch T11, a drain coupled to the drain ofthe transistor switch T2, and a source coupled to the source of thetransistor switch T2. The operation of the LCD device 300 according tothe fifth embodiment of the present invention will be explained indetail in the following paragraphs.

Referring to FIG. 13, which depicts a diagram of the nth stage shiftregister unit SR(n) according to a sixth embodiment of the presentinvention. In the sixth embodiment of the present invention, the shiftregister unit SR(n) includes an input end IN(n), an output end OUT(n), afirst pull-down circuit 11, a second pull-down circuit 21, a fastpull-down circuit 35, a pull-up driving circuit 42 and a pull-up circuit51. Having similar structure as the second embodiment, the sixthembodiment of the present invention further includes the fast pull-downcircuit 35. The fast pull-down circuit in the sixth embodiment of thepresent invention includes transistor switches T11 and T12. Thetransistor switch T11 includes a gate coupled to the output end OUT(n+s)of the shift register unit SR(n+s) for receiving the feedback signalFB(n), a drain coupled to the node Q(n), and a source coupled to thevoltage source VSS. The transistor switch T12 includes a gate coupled tothe gate of the transistor switch T11, a drain coupled to the drain ofthe transistor switch T2, and a source coupled to the source of thetransistor switch T2. The operation of the LCD device 300 according tothe sixth embodiment of the present invention will be explained indetail in the following paragraphs.

Referring to FIG. 14, which depicts a diagram of the nth stage shiftregister unit SR(n) according to a seventh embodiment of the presentinvention. In the seventh embodiment of the present invention, the shiftregister unit SR(n) includes an input end IN(n), an output end OUT(n), afirst pull-down circuit 13, a second pull-down circuit 23, a fastpull-down circuit 35, a pull-up driving circuit 41 and a pull-up circuit53. Having similar structure as the third embodiment, the seventhembodiment of the present invention further includes the fast pull-downcircuit 35. The fast pull-down circuit 35 in the seventh embodiment ofthe present invention includes transistor switches T11 and T12. Thetransistor switch T11 includes a gate coupled to the output end OUT(n+s)of the shift register unit SR(n+s) for receiving the feedback signalFB(n), a drain coupled to the node Q(n), and a source coupled to thevoltage source VSS. The transistor switch T12 includes a gate coupled tothe gate of the transistor switch T11, a drain coupled to the drain ofthe transistor switch T2, and a source coupled to the source of thetransistor switch T2. The operation of the LCD device 300 according tothe seventh embodiment of the present invention will be explained indetail in the following paragraphs.

Referring to FIG. 15, which depicts a diagram of the nth stage shiftregister unit SR(n) according to an eighth embodiment of the presentinvention. In the eighth embodiment of the present invention, the shiftregister unit SR(n) includes an input end IN(n), an output end OUT(n), afirst pull-down circuit 13, a second pull-down circuit 23, a fastpull-down circuit 35, a pull-up driving circuit 41 and a pull-up circuit53. Having similar structure as the fourth embodiment, the eighthembodiment of the present invention further includes the fast pull-downcircuit 35. The fast pull-down circuit in the eighth embodiment of thepresent invention includes transistor switches T11 and T12. Thetransistor switch T11 includes a gate coupled to the output end OUT(n+s)of the shift register unit SR(n+s) for receiving the feedback signalFB(n), a drain coupled to the node Q(n), and a source coupled to thevoltage source VSS. The transistor switch T12 includes a gate coupled tothe gate of the transistor switch T11, a drain coupled to the drain ofthe transistor switch T2, and a source coupled to the source of thetransistor switch T2. The operation of the LCD device 300 according tothe eighth embodiment of the present invention will be explained indetail in the following paragraphs.

Referring to FIG. 16, which depicts a timing diagram illustrating theoperation of the shift register 300 according to the fifth througheighth embodiments of the present invention when m=1 and s=1. In theembodiment illustrated in FIG. 16, the low-frequency clock signals CL1,CL2, the high-frequency clock signals CH1-CH4, and the start pulsesignal VST1 are used for driving the shift register 210. The pulse widthof the high-frequency clock signals CH1-CH4 is equal to that of thestart pulse signal VST1, but the high-frequency clock signals CH1-CH4differ in phase. Each clock signal switches between a high voltage levelVgh and a low voltage level Vg1 based on a predetermined frequency,wherein the frequency of the low-frequency clock signals CL1 and CL2 ismuch lower than that of the high-frequency clock signals CH1-CH4 (forexample, the pulse width of the low-frequency clock signals CL1 and CL2can be around 100 times as large as that of the start pulse signal VST1or VST2, or the low-frequency clock signals CL1 and CL2 can switchphases every 100 frames when the high-frequency clock signals CH1-CH4switch phases after each frame), and the low-frequency clock signals CL1and CL2 have opposite phases at the same time. In the fifth througheighth embodiments of the present invention, the LCD device 300generates the nth stage gate driving signal GS(n) according to the(n−1)th stage gate driving signal GS(n−1), while compensates thefeed-through effect according to the (n+1)th stage gate driving signalGS(n+1). During the driving period of the (n+1)th stage shift registerunit SR(n+1), the fast pull-down circuit 35 pulls down the gate drivingsignal GS(n) to the low voltage level Vg1 using the transistor switchT12. The charge coupling caused by the driving period of the shiftregister unit SR(n−1) and influencing the data voltages of the pixelscan thus be compensated.

Referring to FIG. 17, which depicts a timing diagram illustrating theoperation of the shift register 300 according to the fifth througheighth embodiments of the present invention when m=1 and s=2. In theembodiment illustrated in FIG. 17, the LCD device 300 generates the nthstage gate driving signal GS(n) according to the (n−1)th stage gatedriving signal GS(n−1), while compensates the feed-through effectaccording to the (n+2)th stage gate driving signal GS(n+2). During thedriving period of the (n+2)th stage shift register unit SR(n+2), thefast pull-down circuit 35 pulls down the gate driving signal GS(n) tothe low voltage level Vg1 using the transistor switch T12. The chargecoupling caused by the driving period of the shift register unit SR(n−1)and influencing the data voltages of the pixels can thus be compensated.

Referring to FIG. 18, which depicts a timing diagram illustrating theoperation of the shift register 300 according to the fifth througheighth embodiments of the present invention when m=2 and s=2. In theembodiment illustrated in FIG. 18, the LCD device 300 generates the nthstage gate driving signal GS(n) according to the (n−2)th stage gatedriving signal GS(n−2), while compensates the feed-through effectaccording to the (n+2)th stage gate driving signal GS(n+2). During thedriving period of the (n+2)th stage shift register unit SR(n+2), thefast pull-down circuit 35 pulls down the gate driving signal GS(n) tothe low voltage level Vg1 using the transistor switch T12. The chargecoupling caused by the driving period of the shift register unit SR(n−2)and influencing the data voltages of the pixels can thus be compensated.

In the above-mentioned embodiments, the transistor switches T1-T14 caninclude thin film transistor (TFT) switches, or other devices havingsimilar function. The values of m and n are merely used to explain howthe feed-through effect can be compensated, but do not limit the scopeof the present invention.

The present invention increases the lifetime and accuracy of thetransistor switches by driving the shift register with a plurality ofhigh-frequency clock signals and two low-frequency clock signals,thereby providing GOA driving circuits with low power-consumption andhigh reliability. Meanwhile, the transistor switch T12 in the fastpull-down circuit 35 can pull down the gate driving signal GS(n) to thelow voltage level Vg1 during the driving period of the shift registerunit SR(n+s), thereby compensating the charge coupling on the datavoltages of the pixels caused by the driving period of the shiftregister unit SR(n−m).

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A shift register comprising a plurality ofserially-coupled shift register units, an Nth stage shift register unitamong the plurality of shift register units comprising: an input end forreceiving an input voltage; an output end for outputting an outputvoltage; a node; a pull-up driving circuit for transmitting the inputvoltage to the node; a pull-up circuit for providing the output voltageaccording to a first clock signal and the input voltage so that theoutput voltage is at a first voltage level during a driving period ofthe Nth stage shift register unit, wherein the first clock signalswitches between the first voltage level and a second voltage level witha predetermined frequency and the first voltage level is higher than thesecond voltage level; a pull-down circuit for maintaining the outputvoltage at a third voltage level during a period excluding the drivingperiod of the Nth stage shift register unit, wherein the third voltagelevel is higher than the second voltage level; and a fast pull-downcircuit for maintaining a voltage level of the node or the output endaccording to a feedback voltage so that the output voltage is at thesecond voltage level during a driving period of an (N+1)th stage shiftregister unit among the plurality of the shift register units.
 2. Theshift register of claim 1, wherein the pull-down circuit furthermaintains the voltage level of the node according to a second clocksignal and a third clock signal, wherein a frequency of the first clocksignal is higher than a frequency of the second or the third clocksignal.
 3. The shift register of claim 2, wherein the fast pull-downcircuit comprises: a first switch including: a first end coupled to thenode; a second end for receiving a voltage having the third voltagelevel; and a control end for receiving the feedback voltage; and asecond switch including: a first end coupled to the output end; a secondend for receiving the first clock signal; and a control end coupled tothe control end of the first switch.